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Scientists have built a self-organizing system of synthetic particles that assemble into clusters in a way that mimics the complicated organization of flocks of birds or colonies of bacteria. The particles form a “living crystal” that moves, swirls, and adjusts to heal cracks.

But what draws flocks of starlings, schools of fish, or rafts of ants together? Flocking or schooling can be a social behavior. However, the similarities among these phenomena, regardless of the creatures involved, led NYU's Jérémie Palacci and his colleagues to wonder if an underlying physical principle could also govern the organization process.

Moving flocks are considered out-of-equilibrium because the movement of each individual in the group causes the equivalent of a flux of energy in the flock. Self-assembling DNA, on the other hand, snaps together under equilibrium conditions—just mix complementary bases in solution and they'll find each other as they drift through the liquid.

The scientists wondered if they could recreate an out-of-equilibrium system using synthetic particles, ensuring that any aggregation could not be due to social interactions. They built synthetic particles using nanoscale cubes of hematite surrounded by a polymer shell. Each cube moves due to a light-triggered chemical reaction. When the researchers shine blue light on these particles, hydrogen peroxide decomposes on an exposed portion of the metallic cubes. That sets up a gradient of peroxide in the solution, with lower concentrations of peroxide next to the cube and higher concentrations further away.

The metallic particles follow that gradient with essentially random motion. But when two particles get close to each other, their induced gradients interact and draw the particles together. With enough particles in the solution, they cluster into crystals.

These crystals rotate, crack, and reorganize to heal defects. This “living crystal” behavior was quite striking and surprising, Palacci says. And it’s completely due to the out-of-equilibrium nature of the system. When the scientists turn off the blue light, the chemical reaction at the surface of the particles stops, the crystal disintegrates, and individual particles disperse in the solution.

Switching on a blue light causes the particles to spontaneously form complex aggregates. (Courtesy Science/AAAS)

The scientists can also control the movement of the particles and the living crystals with an external magnetic field. Now Palacci says they’re studying how the shape of the particles influences the aggregation.

Self-assembly could be a way to build microscale devices like solar cells. Palacci says this “living crystal” could find interesting applications in materials science—in self-healing materials, for example, or ones that change their physical properties on demand.

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Melissae Fellet
Melissae is obsessed with electrons, atoms and molecules. She writes about chemistry, physics and technology and holds a PhD in chemistry from Washington University in St. Louis. Twitter@mfellet

I was driving home, it was dusk and I'd just descended the hill at going south Sunol on 680. I looked up and thought I was looking at a tornado. But none of the cars ahead were reacting as if it was a tornado.

Then it twisted and convoluted as you see in the video and moved off to the left.

By the time I got to the top of the grade it was far off to the left and too dark to identify.

It was quite a sight. But I could not tell if it was birds, insects, bats or what.

John Connor: Why doesnt it become a bomb or something to get me? The Terminator: It cant form complex machines, guns and explosives have chemicals, moving parts, it doesn't work that way, but it can form solid metal shapes. John Connor: Like what? The Terminator: Knives and stabbing weapons.

If I had a vibrating plastic table and I had balls with small magnets in them wouldn't I see something awful similar to this? Get close enough to N and S to mate and you'll stick. Occasionally you'll break off if you're weak enough.

I was enjoying this article on a fascinating and very cool piece of science all the way up to the last paragraph:

"Self-assembly could be a way to build microscale devices like solar cells. Palacci says this “living crystal” could find interesting applications in materials science—in self-healing materials, for example, or ones that change their physical properties on demand."

This is the sort of thing that gives scientists a bad name. When the research is intrinsically interesting but has no application, journalists should just stop asking the application question, and scientists should stop rising to the bait with this sort of "nonsense application" boilerplate.

Birds fly in the slipstream of those ahead of them. Not just like cyclists would slipstream each other. Their wingbeats are timed too, to maximize the uplift from the efferent vortices. In the v-shape of migrating geese or starlings, we see this example.

If I had a vibrating plastic table and I had balls with small magnets in them wouldn't I see something awful similar to this? Get close enough to N and S to mate and you'll stick. Occasionally you'll break off if you're weak enough.

Your idea would have the balls moving either along the N or S field lines. The swimmers that this article is talking about swim in random directions, based on a chemical reaction. We can use a magnetic field to align their directions as well.

The video is recorded using an 8-bit CCD camera, so the images are all black and white, no colors.

OK, that explains it.

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think that the particles are about 2 micrometers in size.

I work with the people who did this experiment.

Was this filmed through a microscope?

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By the way, I think it's interesting how most of the people who took the time to write a comment clearly did not take the time to read anything about the article let alone the article itself.

Yeah, me too, my initial post was about the starlings, then I read the article.

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Melissae did a great job of writing about the work though. Thanks!

It's a good article, but a few more details would help.

If the movement is caused by a light-driven reaction, what causes movement when the blue light is off? Or will that movement eventually stop? And how does the reaction to blue light differ from the reaction to white (or any non-blue) light?

When the research is intrinsically interesting but has no application, journalists should just stop asking the application question

While I agree that cutting-edge science shouldn't need an immediate application to be noteworthy, I would also be hard-pressed to call Ars a hardcore science site. I think writers generally need to pitch their story to their bosses and including possible applications might help sell the story. Also, if it's not included in the article, you'll most likely be reading possible applications in the comments section anyways.

I commend the author for putting prophetic applications of the research in a little blurb at the end instead of in the headline like other publications do.

Yes, we look at the sample through a powerful microscope lens and then record that image using the camera.

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If the movement is caused by a light-driven reaction, what causes movement when the blue light is off? Or will that movement eventually stop? And how does the reaction to blue light differ from the reaction to white (or any non-blue) light?

I don't know the details of the reaction. The particles are small enough that they experience Brownian diffusion, which occurs as a result of the solvent molecules (water) bombarding the particles from every direction. See here:http://en.wikipedia.org/wiki/Brownian_motion

Brownian diffusion is only relevant for particles that are a micrometer or smaller in size. Too big, and the momentum of the liquid molecules is insufficient to move the particles.

If I had a vibrating plastic table and I had balls with small magnets in them wouldn't I see something awful similar to this? Get close enough to N and S to mate and you'll stick. Occasionally you'll break off if you're weak enough.

Your idea would have the balls moving either along the N or S field lines. The swimmers that this article is talking about swim in random directions, based on a chemical reaction. We can use a magnetic field to align their directions as well.

You're assuming I have an applied mag. field. Irregular particles will exhibit random walk. How is this any different than using a (granted cool) chemical reaction to impart random motion until particles bump into one another and coalesce? Reading the comments above if the particles are 2 microns then the motion is inherently 3D which particles on a surface would not demonstrate. However, the video certainly appeared 2D motion.

The mechanism for the random walk in the article is cool. However, the swarming behavior would be emergent in any system for which random motion transitions to a not 100% attractive force when particles get close enough. My magnetic particles idea is a macro example but a video would look nearly identical.

Yes, we look at the sample through a powerful microscope lens and then record that image using the camera.

OK, that explains the high contrast and unusual looks of the film.

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If the movement is caused by a light-driven reaction, what causes movement when the blue light is off? Or will that movement eventually stop? And how does the reaction to blue light differ from the reaction to white (or any non-blue) light?

I don't know the details of the reaction. The particles are small enough that they experience Brownian diffusion, which occurs as a result of the solvent molecules (water) bombarding the particles from every direction. See here:http://en.wikipedia.org/wiki/Brownian_motion

So the particles are in water, that was not clear from the article. Given that they seemed to be moving in a plane I'd assumed that they were moving on some manner of substrate.

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Brownian diffusion is only relevant for particles that are a micrometer or smaller in size. Too big, and the momentum of the liquid molecules is insufficient to move the particles.

I understand.

And thanx for posting here with additional details. There may be some good stuff in the DIO, but they never work the first day and sometimes not for days. I'm rather out of the habit of checking them. I will try in a few days though, this is very interesting.

Birds fly in the slipstream of those ahead of them. Not just like cyclists would slipstream each other. Their wingbeats are timed too, to maximize the uplift from the efferent vortices. In the v-shape of migrating geese or starlings, we see this example.

Yes, basically the entire flocks only has to pay the price for one wrap-up vortex rather than one each. However, I've always taken issue with the idea that birds fly in unison or are somehow working together. To a large extent the birds are simply reacting to the air pressure distributions on their wings. You'll note the changes in their relative beats as the formation spreads out and comes back together. It's like watching dolphins "surf" on the bow wave of a boat. They're far more sensitive to the pressure gradient on their skin than we are and can use that to their advantage.

Obviously there must be some intelligence to swapping in and out of the lead spot of the chevron, but I think a lot of the characteristics of bird flight are emergent rather than governing.

So you shine light on them, they effervesce isotropically. Effervescence pushes the ball, but on all sides, the effect is netted out. Brownian motion pushes them around. If they don't have any solvent on one side (because they have another ball), then they don't effervesce on that side, (you can't effervesce in another ball), hence net effervescence from that ball pushes it towards the other ball. So obviously you're going to get aggregations. On that scale in a solvent, obviously you get things being pushed out of equilibrium by brownian motion. What am I missing that makes this worth writing (esp. in a paper)?

and have the authors ever seen a flock of birds? They don't have angular momentum around the centre of an aggregation, they don't jiggle, they don't bump into eachother. They swoop and swoosh and all move together in one direction, then change direction suddenly, then get thinner, fatter, disintegrate, reintegrate, and they are fast together not fast at moving apart. etc. For example: https://www.youtube.com/watch?v=XH-groCeKbE

I'm always impressed at the number of cutting edge scientists that find time to post on Ars. Given the number of people that are dismissing this as "obvious" or "boring", clearly the number of PhD's here is quite high.

...By the way, I think it's interesting how most of the people who took the time to write a comment clearly did not take the time to read anything about the article let alone the article itself.

Do you realize the article is behind a paywall? Publishing in Science may look good to some snobbish pen-pusher, but in the end of the day a quality work stands out based on its own merits and citations.

Maybe you could clue your friends in on this new thing, called the Internet, which allows people to find your work even when it's published in a open access, lower-impact journal? Excuse the snark, but way too many scientist are still living in the 20th century.

Seeing the headline and the first picture reminded me of a great Stanisław Lem novel - The Invincible.

Although the discussed in the article is a very early version (v0.0(n)1) of what could be insect-like nanobots and may not end as these depicted in the novel, the latter is still a great read. I recommend it to anyone that finds this news interesting, if not for 'seeing' these swarming insect-like robots that form crystals and 'black monoliths', at least for the raised philosophical questions regarding technological evolution.

I liked very much the quote that @atlcomputech posted and would definitely read the book. It's great and and the same time frightening to watch today's scientific progress and be reminded of the visions great sci-fi authors have had years/decades ago.

I would expect the behavior could be modeled pretty nicely with 2D lattice statistics (images actually remind me of the old BASIC program/screensaver Life). It's probably an already-started graduate project in this research group.

I'm always impressed at the number of cutting edge scientists that find time to post on Ars. Given the number of people that are dismissing this as "obvious" or "boring", clearly the number of PhD's here is quite high.

Now, the challenge is to figure out if I'm being satirical

Actually, I'm sure there are quite a number of PhDs in the audience. Given that most of the time there are at least a couple of replies from people working in related fields of almost all the science articles there's certainly some breadth too.

That said, most of the science articles get a bit more positive reception than this one. While the process by which the authors have demonstrated an ability to create an on-demand random walk which also happens to couple into a near-field potential is quite cool I'm not sure the commentary matches with the reality. I don't know if it was Kyle's writing or the authors' beliefs that this work was representative of "flocking" behaviors or if that was simply the original motivations. However, the results, at least in the video, don't really resemble flocking in any manner. Flocks respond to threats and obstacles more akin to compressible flows with compression and expansion waves propagating through the group to relay information. They don't compress to a finite density and rotate around (as has already been observed). The particles clumped and sometimes fell apart. That just means the clumps are only quasi stable - the random external force can occasionally overcome the binding energy. Yes, to anyone with a background in statistical mechanics and gas dynamics, etc. much of this is pretty obvious.

Let me reiterate: the mechanism is very cool - the results? Not so impressive.

Exactly! Contrast makes it easier to automatically identify the particle positions using a computer.

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So the particles are in water, that was not clear from the article. Given that they seemed to be moving in a plane I'd assumed that they were moving on some manner of substrate.

Yes, they are in water, and on a glass surface. They sink to the glass surface and diffuse around on it.

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And thanx for posting here with additional details. There may be some good stuff in the DIO, but they never work the first day and sometimes not for days. I'm rather out of the habit of checking them. I will try in a few days though, this is very interesting.

Oh yeah, I know what you mean. We just appreciate that you read the news articles on a website like Ars. I think the only people who read the actual publication are physics nerds like us. :-) Thanks for the good questions!

Yes, to anyone with a background in statistical mechanics and gas dynamics, etc. much of this is pretty obvious.

Let me reiterate: the mechanism is very cool - the results? Not so impressive.

I'm not sure how it's obvious that the crystals form. Let alone why they never grow bigger than a finite size. The particles aren't forced to crystallize. They swim around in random directions and just end up near one another to form crystals.